The semiconductor industry currently uses different types of semiconductor-based image sensors that use micro-lenses, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others. These image sensors use micro-lenses to focus electromagnetic radiation onto the photo-conversion device, e.g., a photodiode. Also, these image sensors can use filters to select particular wavelengths of electromagnetic radiation for sensing by the photo-conversion device.
Micro-lenses of an image sensor help increase optical efficiency and reduce cross talk between pixel cells. FIG. 1A shows a portion of a CMOS image sensor pixel cell array 100. The array 100 includes pixel cells 10, each being formed on a substrate 1. Each pixel cell 10 includes a photo-conversion device 12, for example, a photodiode. The illustrated array 100 has micro-lenses 20 that collect and focus light on the photo-conversion devices 12. The array 100 can also include a light shield, e.g., a metal layer 7, to block light intended for one photo-conversion device from reaching other photo-conversion devices of the pixel cells 10.
The array 100 can also include a color filter array 30. The color filter array includes color filters 31a, 31b, 31c, each disposed over a pixel cell 10. Each of the filters 31a, 31b, 31c allows only particular wavelengths of light to pass through to a respective photo-conversion device. Typically, the color filter array 30 is arranged in a repeating Bayer pattern and includes two green color filters 31a, a red color filter 31b, and a blue color filter 31c, arranged as shown in FIG. 1B.
Between the color filter array 30 and the pixel cells 10 is an interlayer dielectric (ILD) region 3. The ILD region 3 typically includes multiple layers of interlayer dielectrics and conductors that form connections between devices of the pixel cells 10 and from the pixel cells 10 to circuitry (not shown) peripheral to the array 100. Between the color filter array 30 and the micro-lenses 20 is a dielectric layer 5.
Conventional optical lenses, including micro-lenses 20, use curved surfaces, either concave or convex, to refract electromagnetic waves to converge or diverge, but cannot focus light onto an area smaller than a square wavelength. Micro-lenses 20 are typically made of an oxide with a positive refractive index of around 1.3 to 1.8. Since materials used in these conventional optical lens elements have a positive refractive index, it is necessary that the micro-lenses 20 have curved surfaces to create a focal point close to the active area of the photo-conversion device 12.
A reduction of the size of the pixel cells 10 allows for a greater density of pixel cells to be arranged in the array 100, desirably increasing the resolution of the array 100. Typically, the size of each micro-lens 20 is correlated to the size of the pixel cells 10. Thus, as the pixel cells 10 decrease in size, the size of each micro-lens 20 must also decrease. Disadvantageously, however, conventional micro-lenses 20 do not scale very well. A reduction in size of micro-lenses 20 is limited by optical and resulting electrical performance. As conventional micro-lenses 20 are scaled to smaller sizes approaching the diffraction limit, the light gathering power, which is an indirect measure of external quantum efficiency, decreases significantly. Accordingly, as pixel cells 10 are scaled, creating micro lenses 20 with desired properties, e.g., curvature, focal length, no or minimal loss due to diffraction and interference, among others, is difficult. Therefore, it would be advantageous to have an improved lens for use in image sensors to allow better scaling of pixel cells and/or enhanced image sensor performance.